In electrography, an electrostatic charge image is formed on a dielectric surface, typically the surface of a photoconductive recording element. Development of this image is commonly achieved by contacting it with a two-component developer comprising a mixture of pigmented resinous particles (known as "toner") and magnetically attractable particles (known as "carrier"). The carrier particles serve as sites against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge opposite to that of the electrostatic image. During contact between the electrostatic image and the developer mixture, the toner particles are stripped from the carrier particles to which they had formerly adhered (via triboelectric forces) by the relatively strong electrostatic forces associated with the charge image. In this manner, the toner particles are deposited on the electrostatic image to render it visible.
It is known in the art to apply developer compositions of the above type to electrostatic images by means of a magnetic applicator which comprises a cylindrical sleeve of non-magnetic material having a magnetic core positioned within. The core usually comprises a plurality of parallel magnetic strips which are arranged around the core surface to present alternative north and south magnetic fields. These fields project radially, through the sleeve, and serve to attract the developer composition to the sleeve's outer surface to form a brushed nap. Either or both the cylindrical sleeve and the magnetic core are rotated with respect to each other to cause the developer to advance from a supply sump to a position in which it contacts the electrostatic image to be developed. After development, the toner-depleted carrier particles are returned to the sump for toner replenishment.
Conventionally, carrier particles made of soft magnetic materials have been employed to carry and deliver the toner particles to the electrostatic image. U.S. Pat. Nos. 4,546,060 to Miskinis et al, and 4,473,029 to Fritz et al, teach the use of hard magnetic materials as carrier particles and an apparatus for the development of electrostatic images utilizing such hard magnetic carrier particles. These patents require that the carrier particles comprise a hard magnetic material exhibiting a coercivity of at least 300 Oersteds when magnetically saturated and an induced magnetic moment of at least 20 EMU/g when in an applied magnetic field of 1000 Oersteds. The terms "hard" and "soft" when referring to magnetic materials have the generally accepted meaning as indicated on page 18 of Introduction to Magnetic Materials by B. D. Cullity published by Addison-Wesley Publishing Company, 1972. These hard magnetic carrier materials represent a great advance over the use of soft magnetic carrier materials in that the speed of development is remarkably increased without experiencing deterioration of the image. Speeds as high as four times the maximum speed utilized in the use of soft magnetic carrier particles have been demonstrated.
The above two mentioned U.S. patents, while generic to all hard magnetic materials having the properties set forth, prefer the hard magnetic ferrites which are compounds of barium and/or strontium such as, BaFe.sub.12 O.sub.19, SrFe.sub.12 O.sub.19 and the magnetic ferrites having the formula MO..sub.6 Fe.sub.2 O.sub.3, where M is barium, strontium or lead as disclosed in U.S. Pat. No. 3,716,630.
In copending U.S. application Ser. No. 561,382, entitled "Interdispersed Two-Phase Ferrite Composite and Electrographic Magnetic Carrier Particles Therefrom", filed Aug. 1, 1990, which is incorporated herein by reference, it is disclosed that the properties of the hard ferrite magnetic carrier particles described in aforementioned U.S. Pat. No. 4,546,060 and U.S. Pat. No. 4,473,029 can be improved by the addition of cobalt, manganese or iron to the particles. As described therein, it was found that the addition of cobalt, manganese or iron to the hard ferrite magnetic particles, results in the formation of a two-phase composite structure which has a magnetic moment that is higher than the magnetic moment of the corresponding hard ferrite magnetic material by itself exclusive of the added cobalt, manganese or iron. This results in a higher rate of flow of developer compositions comprising mixtures of charged toner particles and oppositely charged carrier particles prepared from the two-phase composite materials around the shell of a rotating-core magnetic applicator used to develop electrostatic images of the type disclosed and described in previously mentioned U.S. Pat. Nos. 4,547,060 and 4,473,029. This in turn results in higher development speeds which means that more copies can be produced per unit time.
In addition to facilitating the rapid flow of developer smoothly around the shell of the rotating-core magnetic applicator, the increase in magnetic moment provided by the two-phase composite materials described in aforementioned copending U.S. application Ser. No. 561,382, entitled "Interdispersed Two-Phase Ferrite Composite and Electrographic Magnetic Carrier Particles Therefrom", filed Aug. 1, 1990, results in an increased magnetic attraction between the rotating-core magnetic applicator and magnetic carrier particles formed from the material. This causes the carrier particles to be held more tightly or securely to the applicator shell during core rotation and development which prevents the carrier particles from transferring to and being picked-up by the recording element during development. The transfer of carrier particles from the applicator shell to the image being developed (i.e., "carrier pick-up") is to be avoided because carrier particles which are picked-up by the insulating surface in the toning operation have the effect at toner transfer of holding areas of the transfer surface away from the insulating surface thereby inhibiting toner powder transfer which causes image artifacts such as streaking and "tent-poling" in the transferred, developed and fixed images produced in the copying operation. The occurrence of carrier pick-up is frequently encountered when very small carrier particles, called "fines", are present in the developer composition. These particles, which typically have a particle size of approximately 10 micrometers or less, and more typically 0.1 to 5.0 micrometers, also have significantly lower or weaker magnetic moments due to their smaller particle size than those possessed by larger-sized carrier particles of the same or identical material (i.e., 10 micrometers or greater). "Particle size" as used herein refers to the "average diameters" of the particles. The average diameters of the particles herein are diameters of median particles by volume, i.e., 50 percent of the total volume of the particles is made up of particles that each have a diameter greater than the reported value and 50 percent of the total volume of the particles is made up of particles that each have a diameter less than the reported value. Thus, the ranges for the diameters of the particles in the total volume are reported herein. Because of their extremely small size and hence low magnetic moments, the magnetic attraction between the applicator shell and at least a portion of the small carrier particle fines is insufficient to hold the very small carrier particles on the applicator shell or sleeve during core-rotation and development with the result that the carrier particles tend to migrate readily and freely onto the recording element during development. It is reported in previously mentioned copending U.S. application Ser. No. 561,382, entitled "Interdispersed Two-Phase Ferrite Composite and Electrographic Magnetic Carrier Particles Therefrom", filed Aug. 1, 1990, however, that carrier pick-up can be ameliorated or reduced by as much as 80%, typically 75% to 80%, by utilizing developer compositions containing carrier particles prepared from the two-phase composite materials described therein in the development of electrostatic images in an electrostatographic development apparatus of the type disclosed and described in U.S. Pat. Nos. 4,564,060 and 4,473,029 as compared to developer compositions otherwise identical in makeup except for containing the aforedescribed hard ferrite magnetic carrier particles of the prior art devoid of added cobalt, manganese or iron in place of the two-phase composite carrier particles even-in developer compositions containing carrier particle fines having diameters of 10 micrometers or less.
Of further significance, it is also reported therein that magnetic carrier particles made from such two-phase composite materials have a high coercivity, i.e., at least 300 Oersteds and, typically, about 1000 to 3000 Oersteds when magnetically saturated. A high coercivity is important as it results in better carrier flow, which means that the carrier particles flip 180.degree. on the brush of the rotating-core applicator rather than sliding along the core of the brush which in turn results in a higher charge on the toner and more delivery of the toner to the photoconductor. This is discussed more fully in aforementioned U.S. Pat. No. 4,546,060 and in U.S. Pat. No. 4,855,205 to Saha et al. While these interdispersed two-phase ferrite materials provide for improved carrier particles having an enhanced magnetic moment, the addition of cobalt, manganese or iron to the hard ferrite magnetic carrier particles described in aforementioned U.S. Pat. Nos. 4,546,060 and 4,473,029 typically has been found to cause a reduction in the coercivity of such particles such that the coercivity of the resultant interdispersed two-phase carrier particles usually is somewhat less than that of the corresponding hard ferrite magnetic carrier particles devoid of the added cobalt, manganese or iron. Thus, while the addition of cobalt, manganese or iron on the one hand serves to increase the magnetic moment of the corresponding hard ferrite magnetic carrier particles without the added cobalt, manganese or iron, it also serves, unfortunately, on the other hand, to effectively reduce the coercivity of the hard ferrite magnetic carrier particles. This is undesirable since, as previously discussed, a high coercivity is important as it results in better carrier flow which in turn results in a higher charge on the toner and more delivery of the toner to the photoconductor. Accordingly, it would be highly desirable to be able to further improve the interdispersed two-phase magnetic carrier particles described above by enhancing the coercivity of such particles without effecting (i.e., lowering) the high magnetic moment of the particles so that carrier particles possessing both an enhanced magnetic moment essential to the attainment of high development speeds and an enhanced coercivity essential to the attainment of smooth carrier flow around the shell of the rotating-core magnetic applicator are provided to the art.